Preheat the oven to 350°F, and lightly grease a 7-inch cake pan. Set aside.
In a medium bowl, sift together the flour, cocoa powder, baking soda and salt.
½ cup cake flour, 1 teaspoon cocoa powder, ¼ teaspoon baking soda, ¼ teaspoon salt
In a separate medium bowl, use an electric mixer to cream together the butter and sugar until light and fluffy, about 3 minutes. Add the egg, and mix well to combine.
2 tablespoons unsalted butter, ½ cup granulated sugar, 1 large egg
Add the oil, vanilla, food coloring and vinegar to the butter mixture, and continue to mix on low speed just until combined.
1 tablespoon canola oil, 2 ½ teaspoons red food coloring, 1 teaspoon vanilla extract, ½ teaspoon white vinegar
Add half of the dry ingredients and half of the buttermilk to the wet ingredients. Beat on low speed, then repeat with the remaining dry and wet ingredients. Pour the batter into the prepared pan.
¼ cup buttermilk
Bake for 15-18 minutes or until a toothpick inserted in the center comes out clean. Allow the cake to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.